The present invention relates to dental cements particularly used for restoration of teeth, root canal treatment and periodontal tissue treatment, and more particularly to dental cements for such use which sets in the treated parts without stimulating the dental pulps.
So far zinc phosphate cement, polycarboxylate cement, glass ionomer cement and others are known for dental use, and they are used for particular uses depending upon their properties.
The zinc phosphate cement containing zinc oxide and orthophosphoric acid is well known, and actually has been in wide use over a long period of time. Nowadays it finds many applications in dental treatments. This cement is prepared by mixing a powdery constituent consisting mainly of ZnO and an aqueous constituent consisting of orthophosphoric acid, phosphate and water. When they are mixed, they chemically react with each other to form a hydrate of zinc phosphate, which is considered to surround the zinc oxide particles and grow into a solid mass. The nature and properties of zinc phosphate cement are fully studied and known so that it provides no problem in handling. In fact the zinc phosphate cement in commerce is superior in the hardening action. However one of the disadvantages is that it tends to contract when it hardens, and another is that the surface is likely to dissolve when it is subjected to water. A further disadvantage is that the pH value of the cement decreases until it indicates strong acidity at the initial stage of mixing the powdery and aqueous constituents. If the highly acid cement is used for the dental treatments mentioned above, it is likely to stimulate the tooth pulps and damage the flesh. A considerable care must be taken not to damage the neighboring teeth when such a highly acid cement as zinc phosphate cement is used. It is recommended that varnishes and zinc oxide eugenol-base liners are used.
The polycarboxylate cement in commerce contains a 32 to 42% aqueous solution of polyacrylic acid having a molecular weight of 25,000 to 50,000 and a powdery constituent consisting mainly of ZnO and MgO. For use, the aqueous constituent and powdery constituent are mixed to form a zinc polyacrylate gel in which the unreacted zinc oxide particles are retained, and solidifies. Advantages of the polycarboxylate cement are (1) its strong bond to the enamel and dentin, and (2) that although the mixture likewise exhibits a stronger acidity than the zinc phosphate cement, the high acidity derives from the dissociation of acid but actually the acidity is not detrimentally high. In addition, it has a large molecular weight, so that there is no likelihood of seriously stimulating the tooth pulps. However the disadvantage is that the resulting solidity is lower than is with the zinc phosphate cement, so that the mass cannot withstand the biting pressure over a long period of time.
In order to solve the problem of the polyacrylate cement, glass ionomer cement is proposed, which contains a mixture of an aqueous solution consisting mainly of polyacrylate and a powdery constituent consisting mainly of SiO.sub.2 and Al.sub.2 O.sub.3. This cement is tough and strengthens the teeth owing to the fluoride release. Recently this property is highly appreciated, but, on the other hand, it has been found that at the initial stage the cement is apt to soften in the presence of water, and that it stimulates the dental roots.
In root canal treatments zinc oxide eugenol cement is widely used. However, it has the same drawbacks of stimulating the teeth and lacking compatibility with the tissues.
Accordingly, the present invention aims at overcoming the difficulties pointed out with respect to the known dental cements discussed above. Thus an object of the present invention is to provide a dental cement which does not substantially stimulate the tooth pulps.
A further object of the present invention is to provide a dental cement which is compatible with the